The post-perovskite transition in Fe- and Al-bearing bridgmanite: effects on seismic observables

The major mineral phase of the Earth's lower mantle, (Al,Fe)-bearing bridgmanite, transitions to a seemingly layered structure known as post-perovskite at Earth's deep lower mantle pressure and temperature conditions. Despite extensive experimental investigations and ab initio calculations, there are still some important aspects of this transformation that need clarification. Here, we systematically address this transition in (Al³⁺, Fe³⁺)-, (Al³⁺)-, (Fe²⁺)- and (Fe³⁺)-bearing bridgmanite using ab initio calculations, particularly the phase boundary dependence on the chemistry and acoustic velocity changes across this transformation. These results help us to constrain possible seismic signatures of this phase transition, which is necessary for a better understanding of the nature of the D" region.

While our results are consistent with previous mineral physics studies, we find that the seismic features produced by the post-perovskite transition depend distinctly on the chemical composition of bridgmanite. Therefore, the absence of a D" seismic discontinuity or signature of a double-crossing of the post-perovskite phase boundary have clear implications for the local aggregate chemistry and/or temperature. Moreover, changes in the bulk velocities throughout the phase change exhibit softening anomalies attributed to changes in the volume fractions of co-existing phases across the phase transition.

Acknowledgment: Research supported by NSF awards EAR-1502594 and EAR-1918126.